When my friends and I play we use a variant rule we call "Taking 2/3." We came up with it from a D&D 3.5 variant rule from Unearthed Arcana. In D&D you get a base armor class of 10 which is the average roll on a d20. The variant rule allows you to rolled your armor class every time you are attacked.

Essentially we instituted it backwards so that instead of rolling you can take the "average" roll. Since the standard minifig uses a 4 from a roll of 1d6, which is 2/3, we allow a player to take 2/3 of his maximum possible armor roll.

I actually calculated out all the die roll probabilities, including the critical-success possibility of bonus dice earning bonus dice ad infinitum; in all cases they round to half the number of sides, plus one.